Hardware friendly design for intra mode coding

ABSTRACT

Methods, apparatus, and computer readable storage medium for intra prediction mode coding in video decoding. The method includes receiving a coded video bitstream; constructing a list of intra modes for a block in the coded video bitstream according to a pre-defined rule based on a size of the block; dividing the list of intra modes into a plurality of intra mode sets for the block; extracting, from the coded video bitstream, a set index indicating an intra mode set from the plurality of intra mode sets; extracting, from the coded video bitstream, a mode index indicating an intra prediction mode from the intra mode set; determining the intra prediction mode for the block based on the set index and the mode index; and decoding the coded video bitstream based on the intra prediction mode.

RELATED APPLICATION

This application is a based on and claims the benefit of priority toU.S. Provisional Application No. 63/236,542 filed on Aug. 24, 2021,which is herein incorporated by reference in its entirety.

FIELD OF THE TECHNOLOGY

The present disclosure relates to video coding and/or decodingtechnologies, and in particular, to improved design and signaling ofintra prediction mode coding.

BACKGROUND OF THE DISCLOSURE

This background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent the work is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing of thisapplication, are neither expressly nor impliedly admitted as prior artagainst the present disclosure.

Video coding and decoding can be performed using inter-pictureprediction with motion compensation. Uncompressed digital video caninclude a series of pictures, with each picture having a spatialdimension of, for example, 1920×1080 luminance samples and associatedfull or subsampled chrominance samples. The series of pictures can havea fixed or variable picture rate (alternatively referred to as framerate) of, for example, 60 pictures per second or 60 frames per second.Uncompressed video has specific bitrate requirements for streaming ordata processing. For example, video with a pixel resolution of1920×1080, a frame rate of 60 frames/second, and a chroma subsampling of4:2:0 at 8 bit per pixel per color channel requires close to 1.5 Gbit/sbandwidth. An hour of such video requires more than 600 GBytes ofstorage space.

One purpose of video coding and decoding can be the reduction ofredundancy in the uncompressed input video signal, through compression.Compression can help reduce the aforementioned bandwidth and/or storagespace requirements, in some cases, by two orders of magnitude or more.Both lossless compression and lossy compression, as well as acombination thereof can be employed. Lossless compression refers totechniques where an exact copy of the original signal can bereconstructed from the compressed original signal via a decodingprocess. Lossy compression refers to coding/decoding process whereoriginal video information is not fully retained during coding and notfully recoverable during decoding. When using lossy compression, thereconstructed signal may not be identical to the original signal, butthe distortion between original and reconstructed signals is made smallenough to render the reconstructed signal useful for the intendedapplication albeit some information loss. In the case of video, lossycompression is widely employed in many applications. The amount oftolerable distortion depends on the application. For example, users ofcertain consumer video streaming applications may tolerate higherdistortion than users of cinematic or television broadcastingapplications. The compression ratio achievable by a particular codingalgorithm can be selected or adjusted to reflect various distortiontolerance: higher tolerable distortion generally allows for codingalgorithms that yield higher losses and higher compression ratios.

A video encoder and decoder can utilize techniques from several broadcategories and steps, including, for example, motion compensation,Fourier transform, quantization, and entropy coding.

Video codec technologies can include techniques known as intra coding.In intra coding, sample values are represented without reference tosamples or other data from previously reconstructed reference pictures.In some video codecs, a picture is spatially subdivided into blocks ofsamples. When all blocks of samples are coded in intra mode, thatpicture can be referred to as an intra picture. Intra pictures and theirderivatives such as independent decoder refresh pictures, can be used toreset the decoder state and can, therefore, be used as the first picturein a coded video bitstream and a video session, or as a still image. Thesamples of a block after intra prediction can then be subject to atransform into frequency domain, and the transform coefficients sogenerated can be quantized before entropy coding. Intra predictionrepresents a technique that minimizes sample values in the pre-transformdomain. In some cases, the smaller the DC value after a transform is,and the smaller the AC coefficients are, the fewer the bits that arerequired at a given quantization step size to represent the block afterentropy coding.

Traditional intra coding such as that known from, for example, MPEG-2generation coding technologies, does not use intra prediction. However,some newer video compression technologies include techniques thatattempt coding/decoding of blocks based on, for example, surroundingsample data and/or metadata that are obtained during the encoding and/ordecoding of spatially neighboring, and that precede in decoding orderthe blocks of data being intra coded or decoded. Such techniques arehenceforth called “intra prediction” techniques. Note that in at leastsome cases, intra prediction uses reference data only from the currentpicture under reconstruction and not from other reference pictures.

There can be many different forms of intra prediction. When more thanone of such techniques are available in a given video coding technology,the technique in use can be referred to as an intra prediction mode. Oneor more intra prediction modes may be provided in a particular codec. Incertain cases, modes can have submodes and/or may be associated withvarious parameters, and mode/submode information and intra codingparameters for blocks of video can be coded individually or collectivelyincluded in mode codewords. Which codeword to use for a given mode,submode, and/or parameter combination can have an impact in the codingefficiency gain through intra prediction, and so can the entropy codingtechnology used to translate the codewords into a bitstream.

A certain mode of intra prediction was introduced with H.264, refined inH.265, and further refined in newer coding technologies such as jointexploration model (JEM), versatile video coding (VVC), and benchmark set(BMS). Generally, for intra prediction, a predictor block can be formedusing neighboring sample values that have become available. For example,available values of particular set of neighboring samples along certaindirection and/or lines may be copied into the predictor block. Areference to the direction in use can be coded in the bitstream or mayitself be predicted.

Referring to FIG. 1A, depicted in the lower right is a subset of ninepredictor directions specified in H.265's 33 possible intra predictordirections (corresponding to the 33 angular modes of the 35 intra modesspecified in H.265). The point where the arrows converge (101)represents the sample being predicted. The arrows represent thedirection from which neighboring samples are used to predict the sampleat 101. For example, arrow (102) indicates that sample (101) ispredicted from a neighboring sample or samples to the upper right, at a45 degree angle from the horizontal direction. Similarly, arrow (103)indicates that sample (101) is predicted from a neighboring sample orsamples to the lower left of sample (101), in a 22.5 degree angle fromthe horizontal direction.

Still referring to FIG. 1A, on the top left there is depicted a squareblock (104) of 4×4 samples (indicated by a dashed, boldface line). Thesquare block (104) includes 16 samples, each labeled with an “S”, itsposition in the Y dimension (e.g., row index) and its position in the Xdimension (e.g., column index). For example, sample S21 is the secondsample in the Y dimension (from the top) and the first (from the left)sample in the X dimension. Similarly, sample S44 is the fourth sample inblock (104) in both the Y and X dimensions. As the block is 4×4 samplesin size, S44 is at the bottom right. Further shown are example referencesamples that follow a similar numbering scheme. A reference sample islabeled with an R, its Y position (e.g., row index) and X position(column index) relative to block (104). In both H.264 and H.265,prediction samples adjacently neighboring the block under reconstructionare used.

Intra picture prediction of block 104 may begin by copying referencesample values from the neighboring samples according to a signaledprediction direction. For example, assuming that the coded videobitstream includes signaling that, for this block 104, indicates aprediction direction of arrow (102)—that is, samples are predicted froma prediction sample or samples to the upper right, at a 45-degree anglefrom the horizontal direction. In such a case, samples S41, S32, S23,and S14 are predicted from the same reference sample R05. Sample S44 isthen predicted from reference sample R08.

In certain cases, the values of multiple reference samples may becombined, for example through interpolation, in order to calculate areference sample; especially when the directions are not evenlydivisible by 45 degrees.

The number of possible directions has increased as video codingtechnology has continued to develop. In H.264 (year 2003), for example,nine different direction are available for intra prediction. Thatincreased to 33 in H.265 (year 2013), and JEM/VVC/BMS, at the time ofthis disclosure, can support up to 65 directions. Experimental studieshave been conducted to help identify the most suitable intra predictiondirections, and certain techniques in the entropy coding may be used toencode those most suitable directions in a small number of bits,accepting a certain bit penalty for directions. Further, the directionsthemselves can sometimes be predicted from neighboring directions usedin the intra prediction of the neighboring blocks that have beendecoded.

FIG. 1B shows a schematic (180) that depicts 65 intra predictiondirections according to JEM to illustrate the increasing number ofprediction directions in various encoding technologies developed overtime.

The manner for mapping of bits representing intra prediction directionsto the prediction directions in the coded video bitstream may vary fromvideo coding technology to video coding technology; and can range, forexample, from simple direct mappings of prediction direction to intraprediction mode, to codewords, to complex adaptive schemes involvingmost probable modes, and similar techniques. In all cases, however,there can be certain directions for intro prediction that arestatistically less likely to occur in video content than certain otherdirections. As the goal of video compression is the reduction ofredundancy, those less likely directions will, in a well-designed videocoding technology, may be represented by a larger number of bits thanmore likely directions.

Inter picture prediction, or inter prediction may be based on motioncompensation. In motion compensation, sample data from a previouslyreconstructed picture or part thereof (reference picture), after beingspatially shifted in a direction indicated by a motion vector (MVhenceforth), may be used for a prediction of a newly reconstructedpicture or picture part (e.g., a block). In some cases, the referencepicture can be the same as the picture currently under reconstruction.MVs may have two dimensions X and Y, or three dimensions, with the thirddimension being an indication of the reference picture in use (akin to atime dimension).

In some video compression techniques, a current MV applicable to acertain area of sample data can be predicted from other MVs, for examplefrom those other MVs that are related to other areas of the sample datathat are spatially adjacent to the area under reconstruction and precedethe current MV in decoding order. Doing so can substantially reduce theoverall amount of data required for coding the MVs by relying onremoving redundancy in correlated MVs, thereby increasing compressionefficiency. MV prediction can work effectively, for example, becausewhen coding an input video signal derived from a camera (known asnatural video) there is a statistical likelihood that areas larger thanthe area to which a single MV is applicable move in a similar directionin the video sequence and, therefore, can in some cases be predictedusing a similar motion vector derived from MVs of neighboring area. Thatresults in the actual MV for a given area to be similar or identical tothe MV predicted from the surrounding MVs. Such an MV in turn may berepresented, after entropy coding, in a smaller number of bits than whatwould be used if the MV is coded directly rather than predicted from theneighboring MV(s). In some cases, MV prediction can be an example oflossless compression of a signal (namely: the MVs) derived from theoriginal signal (namely: the sample stream). In other cases, MVprediction itself can be lossy, for example because of rounding errorswhen calculating a predictor from several surrounding MVs.

Various MV prediction mechanisms are described in H.265/HEVC (ITU-T Rec.H.265, “High Efficiency Video Coding”, December 2016). Out of the manyMV prediction mechanisms that H.265 specifies, described below is atechnique henceforth referred to as “spatial merge”.

Specifically, referring to FIG. 2 , a current block (201) comprisessamples that have been found by the encoder during the motion searchprocess to be predictable from a previous block of the same size thathas been spatially shifted. Instead of coding that MV directly, the MVcan be derived from metadata associated with one or more referencepictures, for example from the most recent (in decoding order) referencepicture, using the MV associated with either one of five surroundingsamples, denoted A0, A1, and B0, B1, B2 (202 through 206, respectively).In H.265, the MV prediction can use predictors from the same referencepicture that the neighboring block uses.

SUMMARY

The present disclosure describes various embodiments of methods,apparatus, and computer-readable storage medium for video encodingand/or decoding.

According to one aspect, an embodiment of the present disclosureprovides a method for intra prediction mode coding in video decoding.The method includes receiving, by a device, a coded video bitstream. Thedevice includes a memory storing instructions and a processor incommunication with the memory. The method also includes constructing, bythe device, a list of intra modes for a block in the coded videobitstream according to a pre-defined rule based on a size of the block,by the device, the list of intra modes into a plurality of intra modesets for the block; extracting, by the device from the coded videobitstream, a set index indicating an intra mode set from the pluralityof intra mode sets; extracting, by the device from the coded videobitstream, a mode index indicating an intra prediction mode from theintra mode set; determining, by the device, the intra prediction modefor the block based on the set index and the mode index; and decoding,by the device, the coded video bitstream based on the intra predictionmode.

According to another aspect, an embodiment of the present disclosureprovides an apparatus for video encoding and/or decoding. The apparatusincludes a memory storing instructions; and a processor in communicationwith the memory. When the processor executes the instructions, theprocessor is configured to cause the apparatus to perform the abovemethods for video decoding and/or encoding.

In another aspect, an embodiment of the present disclosure providesnon-transitory computer-readable mediums storing instructions which whenexecuted by a computer for video decoding and/or encoding cause thecomputer to perform the above methods for video decoding and/orencoding.

The above and other aspects and their implementations are described ingreater detail in the drawings, the descriptions, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, the nature, and various advantages of the disclosedsubject matter will be more apparent from the following detaileddescription and the accompanying drawings.

FIG. 1A shows a schematic illustration of an exemplary subset of intraprediction directional modes.

FIG. 1B shows an illustration of exemplary intra prediction directions.

FIG. 2 shows a schematic illustration of a current block and itssurrounding spatial merge candidates for motion vector prediction in oneexample.

FIG. 3 shows a schematic illustration of a simplified block diagram of acommunication system (300) in accordance with an example embodiment.

FIG. 4 shows a schematic illustration of a simplified block diagram of acommunication system (400) in accordance with an example embodiment.

FIG. 5 shows a schematic illustration of a simplified block diagram of avideo decoder in accordance with an example embodiment.

FIG. 6 shows a schematic illustration of a simplified block diagram of avideo encoder in accordance with an example embodiment.

FIG. 7 shows a block diagram of a video encoder in accordance withanother example embodiment.

FIG. 8 shows a block diagram of a video decoder in accordance withanother example embodiment.

FIG. 9 shows directional intra prediction modes according to exampleembodiments of the disclosure.

FIG. 10 shows non-directional intra prediction modes according toexample embodiments of the disclosure.

FIG. 11 shows recursive intra prediction modes according to exampleembodiments of the disclosure.

FIG. 12 shows an intra prediction scheme based on various referencelines according to example embodiments of the disclosure.

FIG. 13 shows an offset-based refinement for intra prediction accordingto example embodiments of the disclosure.

FIG. 14A shows another diagram of offset-based refinement for intraprediction according to example embodiments of the disclosure.

FIG. 14B shows another diagram of offset-based refinement for intraprediction according to example embodiments of the disclosure.

FIG. 15 shows flow charts of a method according to an example embodimentof

the disclosure.

FIG. 16 shows a schematic illustration of a computer system inaccordance with example embodiments of the disclosure.

DETAILED DESCRIPTION OF THE DISCLOSURE

The invention will now be described in detail hereinafter with referenceto the accompanied drawings, which form a part of the present invention,and which show, by way of illustration, specific examples ofembodiments. Please note that the invention may, however, be embodied ina variety of different forms and, therefore, the covered or claimedsubject matter is intended to be construed as not being limited to anyof the embodiments to be set forth below. Please also note that theinvention may be embodied as methods, devices, components, or systems.Accordingly, embodiments of the invention may, for example, take theform of hardware, software, firmware or any combination thereof.

Throughout the specification and claims, terms may have nuanced meaningssuggested or implied in context beyond an explicitly stated meaning. Thephrase “in one embodiment” or “in some embodiments” as used herein doesnot necessarily refer to the same embodiment and the phrase “in anotherembodiment” or “in other embodiments” as used herein does notnecessarily refer to a different embodiment. Likewise, the phrase “inone implementation” or “in some implementations” as used herein does notnecessarily refer to the same implementation and the phrase “in anotherimplementation” or “in other implementations” as used herein does notnecessarily refer to a different implementation. It is intended, forexample, that claimed subject matter includes combinations of exemplaryembodiments/implementations in whole or in part.

In general, terminology may be understood at least in part from usage incontext. For example, terms, such as “and”, “or”, or “and/or,” as usedherein may include a variety of meanings that may depend at least inpart upon the context in which such terms are used. Typically, “or” ifused to associate a list, such as A, B or C, is intended to mean A, B,and C, here used in the inclusive sense, as well as A, B or C, here usedin the exclusive sense. In addition, the term “one or more” or “at leastone” as used herein, depending at least in part upon context, may beused to describe any feature, structure, or characteristic in a singularsense or may be used to describe combinations of features, structures orcharacteristics in a plural sense. Similarly, terms, such as “a”, “an”,or “the”, again, may be understood to convey a singular usage or toconvey a plural usage, depending at least in part upon context. Inaddition, the term “based on” or “determined by” may be understood asnot necessarily intended to convey an exclusive set of factors and may,instead, allow for existence of additional factors not necessarilyexpressly described, again, depending at least in part on context.

FIG. 3 illustrates a simplified block diagram of a communication system(300) according to an embodiment of the present disclosure. Thecommunication system (300) includes a plurality of terminal devices thatcan communicate with each other, via, for example, a network (350). Forexample, the communication system (300) includes a first pair ofterminal devices (310) and (320) interconnected via the network (350).In the example of FIG. 3 , the first pair of terminal devices (310) and(320) may perform unidirectional transmission of data. For example, theterminal device (310) may code video data (e.g., of a stream of videopictures that are captured by the terminal device (310)) fortransmission to the other terminal device (320) via the network (350).The encoded video data can be transmitted in the form of one or morecoded video bitstreams. The terminal device (320) may receive the codedvideo data from the network (350), decode the coded video data torecover the video pictures and display the video pictures according tothe recovered video data. Unidirectional data transmission may beimplemented in media serving applications and the like.

In another example, the communication system (300) includes a secondpair of terminal devices (330) and (340) that perform bidirectionaltransmission of coded video data that may be implemented, for example,during a videoconferencing application. For bidirectional transmissionof data, in an example, each terminal device of the terminal devices(330) and (340) may code video data (e.g., of a stream of video picturesthat are captured by the terminal device) for transmission to the otherterminal device of the terminal devices (330) and (340) via the network(350). Each terminal device of the terminal devices (330) and (340) alsomay receive the coded video data transmitted by the other terminaldevice of the terminal devices (330) and (340), and may decode the codedvideo data to recover the video pictures and may display the videopictures at an accessible display device according to the recoveredvideo data.

In the example of FIG. 3 , the terminal devices (310), (320), (330) and(340) may be implemented as servers, personal computers and smart phonesbut the applicability of the underlying principles of the presentdisclosure may not be so limited. Embodiments of the present disclosuremay be implemented in desktop computers, laptop computers, tabletcomputers, media players, wearable computers, dedicated videoconferencing equipment, and/or the like. The network (350) representsany number or types of networks that convey coded video data among theterminal devices (310), (320), (330) and (340), including for examplewireline (wired) and/or wireless communication networks. Thecommunication network (350) may exchange data in circuit-switched,packet-switched, and/or other types of channels. Representative networksinclude telecommunications networks, local area networks, wide areanetworks and/or the Internet. For the purposes of the presentdiscussion, the architecture and topology of the network (350) may beimmaterial to the operation of the present disclosure unless explicitlyexplained herein.

FIG. 4 illustrates, as an example for an application for the disclosedsubject matter, a placement of a video encoder and a video decoder in avideo streaming environment. The disclosed subject matter may be equallyapplicable to other video applications, including, for example, videoconferencing, digital TV broadcasting, gaming, virtual reality, storageof compressed video on digital media including CD, DVD, memory stick andthe like, and so on.

A video streaming system may include a video capture subsystem (413)that can include a video source (401), e.g., a digital camera, forcreating a stream of video pictures or images (402) that areuncompressed. In an example, the stream of video pictures (402) includessamples that are recorded by a digital camera of the video source 401.The stream of video pictures (402), depicted as a bold line to emphasizea high data volume when compared to encoded video data (404) (or codedvideo bitstreams), can be processed by an electronic device (420) thatincludes a video encoder (403) coupled to the video source (401). Thevideo encoder (403) can include hardware, software, or a combinationthereof to enable or implement aspects of the disclosed subject matteras described in more detail below. The encoded video data (404) (orencoded video bitstream (404)), depicted as a thin line to emphasize alower data volume when compared to the stream of uncompressed videopictures (402), can be stored on a streaming server (405) for future useor directly to downstream video devices (not shown). One or morestreaming client subsystems, such as client subsystems (406) and (408)in FIG. 4 can access the streaming server (405) to retrieve copies (407)and (409) of the encoded video data (404). A client subsystem (406) caninclude a video decoder (410), for example, in an electronic device(430). The video decoder (410) decodes the incoming copy (407) of theencoded video data and creates an outgoing stream of video pictures(411) that are uncompressed and that can be rendered on a display (412)(e.g., a display screen) or other rendering devices (not depicted). Thevideo decoder 410 may be configured to perform some or all of thevarious functions described in this disclosure. In some streamingsystems, the encoded video data (404), (407), and (409) (e.g., videobitstreams) can be encoded according to certain video coding/compressionstandards. Examples of those standards include ITU-T RecommendationH.265. In an example, a video coding standard under development isinformally known as Versatile Video Coding (VVC). The disclosed subjectmatter may be used in the context of VVC, and other video codingstandards.

It is noted that the electronic devices (420) and (430) can includeother components (not shown). For example, the electronic device (420)can include a video decoder (not shown) and the electronic device (430)can include a video encoder (not shown) as well.

FIG. 5 shows a block diagram of a video decoder (510) according to anyembodiment of the present disclosure below. The video decoder (510) canbe included in an electronic device (530). The electronic device (530)can include a receiver (531) (e.g., receiving circuitry). The videodecoder (510) can be used in place of the video decoder (410) in theexample of FIG. 4 .

The receiver (531) may receive one or more coded video sequences to bedecoded by the video decoder (510). In the same or another embodiment,one coded video sequence may be decoded at a time, where the decoding ofeach coded video sequence is independent from other coded videosequences. Each video sequence may be associated with multiple videoframes or images. The coded video sequence may be received from achannel (501), which may be a hardware/software link to a storage devicewhich stores the encoded video data or a streaming source whichtransmits the encoded video data. The receiver (531) may receive theencoded video data with other data such as coded audio data and/orancillary data streams, that may be forwarded to their respectiveprocessing circuitry (not depicted). The receiver (531) may separate thecoded video sequence from the other data. To combat network jitter, abuffer memory (515) may be disposed in between the receiver (531) and anentropy decoder/parser (520) (“parser (520)” henceforth). In certainapplications, the buffer memory (515) may be implemented as part of thevideo decoder (510). In other applications, it can be outside of andseparate from the video decoder (510) (not depicted). In still otherapplications, there can be a buffer memory (not depicted) outside of thevideo decoder (510) for the purpose of, for example, combating networkjitter, and there may be another additional buffer memory (515) insidethe video decoder (510), for example to handle playback timing. When thereceiver (531) is receiving data from a store/forward device ofsufficient bandwidth and controllability, or from an isosynchronousnetwork, the buffer memory (515) may not be needed, or can be small. Foruse on best-effort packet networks such as the Internet, the buffermemory (515) of sufficient size may be required, and its size can becomparatively large. Such buffer memory may be implemented with anadaptive size, and may at least partially be implemented in an operatingsystem or similar elements (not depicted) outside of the video decoder(510).

The video decoder (510) may include the parser (520) to reconstructsymbols (521) from the coded video sequence. Categories of those symbolsinclude information used to manage operation of the video decoder (510),and potentially information to control a rendering device such asdisplay (512) (e.g., a display screen) that may or may not an integralpart of the electronic device (530) but can be coupled to the electronicdevice (530), as is shown in FIG. 5 . The control information for therendering device(s) may be in the form of Supplemental EnhancementInformation (SEI messages) or Video Usability Information (VUI)parameter set fragments (not depicted). The parser (520) mayparse/entropy-decode the coded video sequence that is received by theparser (520). The entropy coding of the coded video sequence can be inaccordance with a video coding technology or standard, and can followvarious principles, including variable length coding, Huffman coding,arithmetic coding with or without context sensitivity, and so forth. Theparser (520) may extract from the coded video sequence, a set ofsubgroup parameters for at least one of the subgroups of pixels in thevideo decoder, based upon at least one parameter corresponding to thesubgroups. The subgroups can include Groups of Pictures (GOPs),pictures, tiles, slices, macroblocks, Coding Units (CUs), blocks,Transform Units (TUs), Prediction Units (PUs) and so forth. The parser(520) may also extract from the coded video sequence information such astransform coefficients (e.g., Fourier transform coefficients), quantizerparameter values, motion vectors, and so forth.

The parser (520) may perform an entropy decoding/parsing operation onthe video sequence received from the buffer memory (515), so as tocreate symbols (521).

Reconstruction of the symbols (521) can involve multiple differentprocessing or functional units depending on the type of the coded videopicture or parts thereof (such as: inter and intra picture, inter andintra block), and other factors. The units that are involved and howthey are involved may be controlled by the subgroup control informationthat was parsed from the coded video sequence by the parser (520). Theflow of such subgroup control information between the parser (520) andthe multiple processing or functional units below is not depicted forsimplicity.

Beyond the functional blocks already mentioned, the video decoder (510)can be conceptually subdivided into a number of functional units asdescribed below. In a practical implementation operating undercommercial constraints, many of these functional units interact closelywith each other and can, at least partly, be integrated with oneanother. However, for the purpose of describing the various functions ofthe disclosed subject matter with clarity, the conceptual subdivisioninto the functional units is adopted in the disclosure below.

A first unit may include the scaler/inverse transform unit (551). Thescaler/inverse transform unit (551) may receive a quantized transformcoefficient as well as control information, including informationindicating which type of inverse transform to use, block size,quantization factor/parameters, quantization scaling matrices, and thelie as symbol(s) (521) from the parser (520). The scaler/inversetransform unit (551) can output blocks comprising sample values that canbe input into aggregator (555).

In some cases, the output samples of the scaler/inverse transform (551)can pertain to an intra coded block, i.e., a block that does not usepredictive information from previously reconstructed pictures, but canuse predictive information from previously reconstructed parts of thecurrent picture. Such predictive information can be provided by an intrapicture prediction unit (552). In some cases, the intra pictureprediction unit (552) may generate a block of the same size and shape ofthe block under reconstruction using surrounding block information thatis already reconstructed and stored in the current picture buffer (558).The current picture buffer (558) buffers, for example, partlyreconstructed current picture and/or fully reconstructed currentpicture. The aggregator (555), in some implementations, may add, on aper sample basis, the prediction information the intra prediction unit(552) has generated to the output sample information as provided by thescaler/inverse transform unit (551).

In other cases, the output samples of the scaler/inverse transform unit(551) can pertain to an inter coded, and potentially motion compensatedblock. In such a case, a motion compensation prediction unit (553) canaccess reference picture memory (557) to fetch samples used forinter-picture prediction. After motion compensating the fetched samplesin accordance with the symbols (521) pertaining to the block, thesesamples can be added by the aggregator (555) to the output of thescaler/inverse transform unit (551) (output of unit 551 may be referredto as the residual samples or residual signal) so as to generate outputsample information. The addresses within the reference picture memory(557) from where the motion compensation prediction unit (553) fetchesprediction samples can be controlled by motion vectors, available to themotion compensation prediction unit (553) in the form of symbols (521)that can have, for example X, Y components (shift), and referencepicture components (time). Motion compensation may also includeinterpolation of sample values as fetched from the reference picturememory (557) when sub-sample exact motion vectors are in use, and mayalso be associated with motion vector prediction mechanisms, and soforth.

The output samples of the aggregator (555) can be subject to variousloop filtering techniques in the loop filter unit (556). Videocompression technologies can include in-loop filter technologies thatare controlled by parameters included in the coded video sequence (alsoreferred to as coded video bitstream) and made available to the loopfilter unit (556) as symbols (521) from the parser (520), but can alsobe responsive to meta-information obtained during the decoding ofprevious (in decoding order) parts of the coded picture or coded videosequence, as well as responsive to previously reconstructed andloop-filtered sample values. Several type of loop filters may beincluded as part of the loop filter unit 556 in various orders, as willbe described in further detail below.

The output of the loop filter unit (556) can be a sample stream that canbe output to the rendering device (512) as well as stored in thereference picture memory (557) for use in future inter-pictureprediction.

Certain coded pictures, once fully reconstructed, can be used asreference pictures for future inter-picture prediction. For example,once a coded picture corresponding to a current picture is fullyreconstructed and the coded picture has been identified as a referencepicture (by, for example, the parser (520)), the current picture buffer(558) can become a part of the reference picture memory (557), and afresh current picture buffer can be reallocated before commencing thereconstruction of the following coded picture.

The video decoder (510) may perform decoding operations according to apredetermined video compression technology adopted in a standard, suchas ITU-T Rec. H.265. The coded video sequence may conform to a syntaxspecified by the video compression technology or standard being used, inthe sense that the coded video sequence adheres to both the syntax ofthe video compression technology or standard and the profiles asdocumented in the video compression technology or standard.Specifically, a profile can select certain tools from all the toolsavailable in the video compression technology or standard as the onlytools available for use under that profile. To be standard-compliant,the complexity of the coded video sequence may be within bounds asdefined by the level of the video compression technology or standard. Insome cases, levels restrict the maximum picture size, maximum framerate, maximum reconstruction sample rate (measured in, for examplemegasamples per second), maximum reference picture size, and so on.Limits set by levels can, in some cases, be further restricted throughHypothetical Reference Decoder (HRD) specifications and metadata for HRDbuffer management signaled in the coded video sequence.

In some example embodiments, the receiver (531) may receive additional(redundant) data with the encoded video. The additional data may beincluded as part of the coded video sequence(s). The additional data maybe used by the video decoder (510) to properly decode the data and/or tomore accurately reconstruct the original video data. Additional data canbe in the form of, for example, temporal, spatial, or signal noise ratio(SNR) enhancement layers, redundant slices, redundant pictures, forwarderror correction codes, and so on.

FIG. 6 shows a block diagram of a video encoder (603) according to anexample embodiment of the present disclosure. The video encoder (603)may be included in an electronic device (620). The electronic device(620) may further include a transmitter (640) (e.g., transmittingcircuitry). The video encoder (603) can be used in place of the videoencoder (403) in the example of FIG. 4 .

The video encoder (603) may receive video samples from a video source(601) (that is not part of the electronic device (620) in the example ofFIG. 6 ) that may capture video image(s) to be coded by the videoencoder (603). In another example, the video source (601) may beimplemented as a portion of the electronic device (620).

The video source (601) may provide the source video sequence to be codedby the video encoder (603) in the form of a digital video sample streamthat can be of any suitable bit depth (for example: 8 bit, 10 bit, 12bit, . . . ), any colorspace (for example, BT.601 YCrCb, RGB, XYZ . . .), and any suitable sampling structure (for example YCrCb 4:2:0, YCrCb4:4:4). In a media serving system, the video source (601) may be astorage device capable of storing previously prepared video. In avideoconferencing system, the video source (601) may be a camera thatcaptures local image information as a video sequence. Video data may beprovided as a plurality of individual pictures or images that impartmotion when viewed in sequence. The pictures themselves may be organizedas a spatial array of pixels, wherein each pixel can comprise one ormore samples depending on the sampling structure, color space, and thelike being in use. A person having ordinary skill in the art can readilyunderstand the relationship between pixels and samples. The descriptionbelow focuses on samples.

According to some example embodiments, the video encoder (603) may codeand compress the pictures of the source video sequence into a codedvideo sequence (643) in real time or under any other time constraints asrequired by the application. Enforcing appropriate coding speedconstitutes one function of a controller (650). In some embodiments, thecontroller (650) may be functionally coupled to and control otherfunctional units as described below. The coupling is not depicted forsimplicity. Parameters set by the controller (650) can include ratecontrol related parameters (picture skip, quantizer, lambda value ofrate-distortion optimization techniques, . . . ), picture size, group ofpictures (GOP) layout, maximum motion vector search range, and the like.The controller (650) can be configured to have other suitable functionsthat pertain to the video encoder (603) optimized for a certain systemdesign.

In some example embodiments, the video encoder (603) may be configuredto operate in a coding loop. As an oversimplified description, in anexample, the coding loop can include a source coder (630) (e.g.,responsible for creating symbols, such as a symbol stream, based on aninput picture to be coded, and a reference picture(s)), and a (local)decoder (633) embedded in the video encoder (603). The decoder (633)reconstructs the symbols to create the sample data in a similar manneras a (remote) decoder would create even though the embedded decoder 633process coded video steam by the source coder 630 without entropy coding(as any compression between symbols and coded video bitstream in entropycoding may be lossless in the video compression technologies consideredin the disclosed subject matter). The reconstructed sample stream(sample data) is input to the reference picture memory (634). As thedecoding of a symbol stream leads to bit-exact results independent ofdecoder location (local or remote), the content in the reference picturememory (634) is also bit exact between the local encoder and remoteencoder. In other words, the prediction part of an encoder “sees” asreference picture samples exactly the same sample values as a decoderwould “see” when using prediction during decoding. This fundamentalprinciple of reference picture synchronicity (and resulting drift, ifsynchronicity cannot be maintained, for example because of channelerrors) is used to improve coding quality.

The operation of the “local” decoder (633) can be the same as of a“remote” decoder, such as the video decoder (510), which has alreadybeen described in detail above in conjunction with FIG. 5 . Brieflyreferring also to FIG. 5 , however, as symbols are available andencoding/decoding of symbols to a coded video sequence by an entropycoder (645) and the parser (520) can be lossless, the entropy decodingparts of the video decoder (510), including the buffer memory (515), andparser (520) may not be fully implemented in the local decoder (633) inthe encoder.

An observation that can be made at this point is that any decodertechnology except the parsing/entropy decoding that may only be presentin a decoder also may necessarily need to be present, in substantiallyidentical functional form, in a corresponding encoder. For this reason,the disclosed subject matter may at times focus on decoder operation,which allies to the decoding portion of the encoder. The description ofencoder technologies can thus be abbreviated as they are the inverse ofthe comprehensively described decoder technologies. Only in certainareas or aspects a more detail description of the encoder is providedbelow.

During operation in some example implementations, the source coder (630)may perform motion compensated predictive coding, which codes an inputpicture predictively with reference to one or more previously codedpicture from the video sequence that were designated as “referencepictures.” In this manner, the coding engine (632) codes differences (orresidue) in the color channels between pixel blocks of an input pictureand pixel blocks of reference picture(s) that may be selected asprediction reference(s) to the input picture.

The local video decoder (633) may decode coded video data of picturesthat may be designated as reference pictures, based on symbols createdby the source coder (630). Operations of the coding engine (632) mayadvantageously be lossy processes. When the coded video data may bedecoded at a video decoder (not shown in FIG. 6 ), the reconstructedvideo sequence typically may be a replica of the source video sequencewith some errors. The local video decoder (633) replicates decodingprocesses that may be performed by the video decoder on referencepictures and may cause reconstructed reference pictures to be stored inthe reference picture cache (634). In this manner, the video encoder(603) may store copies of reconstructed reference pictures locally thathave common content as the reconstructed reference pictures that will beobtained by a far-end (remote) video decoder (absent transmissionerrors).

The predictor (635) may perform prediction searches for the codingengine (632). That is, for a new picture to be coded, the predictor(635) may search the reference picture memory (634) for sample data (ascandidate reference pixel blocks) or certain metadata such as referencepicture motion vectors, block shapes, and so on, that may serve as anappropriate prediction reference for the new pictures. The predictor(635) may operate on a sample block-by-pixel block basis to findappropriate prediction references. In some cases, as determined bysearch results obtained by the predictor (635), an input picture mayhave prediction references drawn from multiple reference pictures storedin the reference picture memory (634).

The controller (650) may manage coding operations of the source coder(630), including, for example, setting of parameters and subgroupparameters used for encoding the video data.

Output of all aforementioned functional units may be subjected toentropy coding in the entropy coder (645). The entropy coder (645)translates the symbols as generated by the various functional units intoa coded video sequence, by lossless compression of the symbols accordingto technologies such as Huffman coding, variable length coding,arithmetic coding, and so forth.

The transmitter (640) may buffer the coded video sequence(s) as createdby the entropy coder (645) to prepare for transmission via acommunication channel (660), which may be a hardware/software link to astorage device which would store the encoded video data. The transmitter(640) may merge coded video data from the video coder (603) with otherdata to be transmitted, for example, coded audio data and/or ancillarydata streams (sources not shown).

The controller (650) may manage operation of the video encoder (603).During coding, the controller (650) may assign to each coded picture acertain coded picture type, which may affect the coding techniques thatmay be applied to the respective picture. For example, pictures oftenmay be assigned as one of the following picture types:

An Intra Picture (I picture) may be one that may be coded and decodedwithout using any other picture in the sequence as a source ofprediction. Some video codecs allow for different types of intrapictures, including, for example Independent Decoder Refresh (“IDR”)Pictures. A person having ordinary skill in the art is aware of thosevariants of I pictures and their respective applications and features.

A predictive picture (P picture) may be one that may be coded anddecoded using intra prediction or inter prediction using at most onemotion vector and reference index to predict the sample values of eachblock.

A bi-directionally predictive picture (B Picture) may be one that may becoded and decoded using intra prediction or inter prediction using atmost two motion vectors and reference indices to predict the samplevalues of each block. Similarly, multiple-predictive pictures can usemore than two reference pictures and associated metadata for thereconstruction of a single block.

Source pictures commonly may be subdivided spatially into a plurality ofsample coding blocks (for example, blocks of 4×4, 8×8, 4×8, or 16×16samples each) and coded on a block-by-block basis. Blocks may be codedpredictively with reference to other (already coded) blocks asdetermined by the coding assignment applied to the blocks' respectivepictures. For example, blocks of I pictures may be codednon-predictively or they may be coded predictively with reference toalready coded blocks of the same picture (spatial prediction or intraprediction). Pixel blocks of P pictures may be coded predictively, viaspatial prediction or via temporal prediction with reference to onepreviously coded reference picture. Blocks of B pictures may be codedpredictively, via spatial prediction or via temporal prediction withreference to one or two previously coded reference pictures. The sourcepictures or the intermediate processed pictures may be subdivided intoother types of blocks for other purposes. The division of coding blocksand the other types of blocks may or may not follow the same manner, asdescribed in further detail below.

The video encoder (603) may perform coding operations according to apredetermined video coding technology or standard, such as ITU-T Rec.H.265. In its operation, the video encoder (603) may perform variouscompression operations, including predictive coding operations thatexploit temporal and spatial redundancies in the input video sequence.The coded video data may accordingly conform to a syntax specified bythe video coding technology or standard being used.

In some example embodiments, the transmitter (640) may transmitadditional data with the encoded video. The source coder (630) mayinclude such data as part of the coded video sequence. The additionaldata may comprise temporal/spatial/SNR enhancement layers, other formsof redundant data such as redundant pictures and slices, SEI messages,VUI parameter set fragments, and so on.

A video may be captured as a plurality of source pictures (videopictures) in a temporal sequence. Intra-picture prediction (oftenabbreviated to intra prediction) utilizes spatial correlation in a givenpicture, and inter-picture prediction utilizes temporal or othercorrelation between the pictures. For example, a specific picture underencoding/decoding, which is referred to as a current picture, may bepartitioned into blocks. A block in the current picture, when similar toa reference block in a previously coded and still buffered referencepicture in the video, may be coded by a vector that is referred to as amotion vector. The motion vector points to the reference block in thereference picture, and can have a third dimension identifying thereference picture, in case multiple reference pictures are in use.

In some example embodiments, a bi-prediction technique can be used forinter-picture prediction. According to such bi-prediction technique, tworeference pictures, such as a first reference picture and a secondreference picture that both proceed the current picture in the video indecoding order (but may be in the past or future, respectively, indisplay order) are used. A block in the current picture can be coded bya first motion vector that points to a first reference block in thefirst reference picture, and a second motion vector that points to asecond reference block in the second reference picture. The block can bejointly predicted by a combination of the first reference block and thesecond reference block.

Further, a merge mode technique may be used in the inter-pictureprediction to improve coding efficiency.

According to some example embodiments of the disclosure, predictions,such as inter-picture predictions and intra-picture predictions areperformed in the unit of blocks. For example, a picture in a sequence ofvideo pictures is partitioned into coding tree units (CTU) forcompression, the CTUs in a picture may have the same size, such as 64×64pixels, 32×32 pixels, or 16×16 pixels. In general, a CTU may includethree parallel coding tree blocks (CTBs): one luma CTB and two chromaCTBs. Each CTU can be recursively quadtree split into one or multiplecoding units (CUs). For example, a CTU of 64×64 pixels can be split intoone CU of 64×64 pixels, or 4 CUs of 32×32 pixels. Each of the one ormore of the 32×32 block may be further split into 4 CUs of 16×16 pixels.In some example embodiments, each CU may be analyzed during encoding todetermine a prediction type for the CU among various prediction typessuch as an inter prediction type or an intra prediction type. The CU maybe split into one or more prediction units (PUs) depending on thetemporal and/or spatial predictability. Generally, each PU includes aluma prediction block (PB), and two chroma PBs. In an embodiment, aprediction operation in coding (encoding/decoding) is performed in theunit of a prediction block. The split of a CU into PU (or PBs ofdifferent color channels) may be performed in various spatial pattern. Aluma or chroma PB, for example, may include a matrix of values (e.g.,luma values) for samples, such as 8×8 pixels, 16×16 pixels, 8×16 pixels,16×8 samples, and the like.

FIG. 7 shows a diagram of a video encoder (703) according to anotherexample embodiment of the disclosure. The video encoder (703) isconfigured to receive a processing block (e.g., a prediction block) ofsample values within a current video picture in a sequence of videopictures, and encode the processing block into a coded picture that ispart of a coded video sequence. The example video encoder (703) may beused in place of the video encoder (403) in the FIG. 4 example.

For example, the video encoder (703) receives a matrix of sample valuesfor a processing block, such as a prediction block of 8×8 samples, andthe like. The video encoder (703) then determines whether the processingblock is best coded using intra mode, inter mode, or bi-prediction modeusing, for example, rate-distortion optimization (RDO). When theprocessing block is determined to be coded in intra mode, the videoencoder (703) may use an intra prediction technique to encode theprocessing block into the coded picture; and when the processing blockis determined to be coded in inter mode or bi-prediction mode, the videoencoder (703) may use an inter prediction or bi-prediction technique,respectively, to encode the processing block into the coded picture. Insome example embodiments, a merge mode may be used as a submode of theinter picture prediction where the motion vector is derived from one ormore motion vector predictors without the benefit of a coded motionvector component outside the predictors. In some other exampleembodiments, a motion vector component applicable to the subject blockmay be present. Accordingly, the video encoder (703) may includecomponents not explicitly shown in FIG. 7 , such as a mode decisionmodule, to determine the perdition mode of the processing blocks.

In the example of FIG. 7 , the video encoder (703) includes an interencoder (730), an intra encoder (722), a residue calculator (723), aswitch (726), a residue encoder (724), a general controller (721), andan entropy encoder (725) coupled together as shown in the examplearrangement in FIG. 7 .

The inter encoder (730) is configured to receive the samples of thecurrent block (e.g., a processing block), compare the block to one ormore reference blocks in reference pictures (e.g., blocks in previouspictures and later pictures in display order), generate inter predictioninformation (e.g., description of redundant information according tointer encoding technique, motion vectors, merge mode information), andcalculate inter prediction results (e.g., predicted block) based on theinter prediction information using any suitable technique. In someexamples, the reference pictures are decoded reference pictures that aredecoded based on the encoded video information using the decoding unit633 embedded in the example encoder 620 of FIG. 6 (shown as residualdecoder 728 of FIG. 7 , as described in further detail below).

The intra encoder (722) is configured to receive the samples of thecurrent block (e.g., a processing block), compare the block to blocksalready coded in the same picture, and generate quantized coefficientsafter transform, and in some cases also to generate intra predictioninformation (e.g., an intra prediction direction information accordingto one or more intra encoding techniques). The intra encoder (722) maycalculates intra prediction results (e.g., predicted block) based on theintra prediction information and reference blocks in the same picture.

The general controller (721) may be configured to determine generalcontrol data and control other components of the video encoder (703)based on the general control data. In an example, the general controller(721) determines the prediction mode of the block, and provides acontrol signal to the switch (726) based on the prediction mode. Forexample, when the prediction mode is the intra mode, the generalcontroller (721) controls the switch (726) to select the intra moderesult for use by the residue calculator (723), and controls the entropyencoder (725) to select the intra prediction information and include theintra prediction information in the bitstream; and when the predictionmode for the block is the inter mode, the general controller (721)controls the switch (726) to select the inter prediction result for useby the residue calculator (723), and controls the entropy encoder (725)to select the inter prediction information and include the interprediction information in the bitstream.

The residue calculator (723) may be configured to calculate a difference(residue data) between the received block and prediction results for theblock selected from the intra encoder (722) or the inter encoder (730).The residue encoder (724) may be configured to encode the residue datato generate transform coefficients. For example, the residue encoder(724) may be configured to convert the residue data from a spatialdomain to a frequency domain to generate the transform coefficients. Thetransform coefficients are then subject to quantization processing toobtain quantized transform coefficients. In various example embodiments,the video encoder (703) also includes a residue decoder (728). Theresidue decoder (728) is configured to perform inverse-transform, andgenerate the decoded residue data. The decoded residue data can besuitably used by the intra encoder (722) and the inter encoder (730).For example, the inter encoder (730) can generate decoded blocks basedon the decoded residue data and inter prediction information, and theintra encoder (722) can generate decoded blocks based on the decodedresidue data and the intra prediction information. The decoded blocksare suitably processed to generate decoded pictures and the decodedpictures can be buffered in a memory circuit (not shown) and used asreference pictures.

The entropy encoder (725) may be configured to format the bitstream toinclude the encoded block and perform entropy coding. The entropyencoder (725) is configured to include in the bitstream variousinformation. For example, the entropy encoder (725) may be configured toinclude the general control data, the selected prediction information(e.g., intra prediction information or inter prediction information),the residue information, and other suitable information in thebitstream. When coding a block in the merge submode of either inter modeor bi-prediction mode, there may be no residue information.

FIG. 8 shows a diagram of an example video decoder (810) according toanother embodiment of the disclosure. The video decoder (810) isconfigured to receive coded pictures that are part of a coded videosequence, and decode the coded pictures to generate reconstructedpictures. In an example, the video decoder (810) may be used in place ofthe video decoder (410) in the example of FIG. 4 .

In the example of FIG. 8 , the video decoder (810) includes an entropydecoder (871), an inter decoder (880), a residue decoder (873), areconstruction module (874), and an intra decoder (872) coupled togetheras shown in the example arrangement of FIG. 8 .

The entropy decoder (871) can be configured to reconstruct, from thecoded picture, certain symbols that represent the syntax elements ofwhich the coded picture is made up. Such symbols can include, forexample, the mode in which a block is coded (e.g., intra mode, intermode, bi-predicted mode, merge submode or another submode), predictioninformation (e.g., intra prediction information or inter predictioninformation) that can identify certain sample or metadata used forprediction by the intra decoder (872) or the inter decoder (880),residual information in the form of, for example, quantized transformcoefficients, and the like. In an example, when the prediction mode isthe inter or bi-predicted mode, the inter prediction information isprovided to the inter decoder (880); and when the prediction type is theintra prediction type, the intra prediction information is provided tothe intra decoder (872). The residual information can be subject toinverse quantization and is provided to the residue decoder (873).

The inter decoder (880) may be configured to receive the interprediction information, and generate inter prediction results based onthe inter prediction information.

The intra decoder (872) may be configured to receive the intraprediction information, and generate prediction results based on theintra prediction information.

The residue decoder (873) may be configured to perform inversequantization to extract de-quantized transform coefficients, and processthe de-quantized transform coefficients to convert the residual from thefrequency domain to the spatial domain. The residue decoder (873) mayalso utilize certain control information (to include the QuantizerParameter (QP)) which may be provided by the entropy decoder (871) (datapath not depicted as this may be low data volume control informationonly).

The reconstruction module (874) may be configured to combine, in thespatial domain, the residual as output by the residue decoder (873) andthe prediction results (as output by the inter or intra predictionmodules as the case may be) to form a reconstructed block forming partof the reconstructed picture as part of the reconstructed video. It isnoted that other suitable operations, such as a deblocking operation andthe like, may also be performed to improve the visual quality.

It is noted that the video encoders (403), (603), and (703), and thevideo decoders (410), (510), and (810) can be implemented using anysuitable technique. In some example embodiments, the video encoders(403), (603), and (703), and the video decoders (410), (510), and (810)can be implemented using one or more integrated circuits. In anotherembodiment, the video encoders (403), (603), and (603), and the videodecoders (410), (510), and (810) can be implemented using one or moreprocessors that execute software instructions.

Returning to the intra prediction process, in which samples in a block(e.g., a luma or chroma prediction block, or coding block if not furthersplit into prediction blocks) is predicted by samples of neighboring,next neighboring, or other line or lines, or the combination thereof, togenerate a prediction block. The residual between the actual block beingcoded and the prediction block may then be processed via transformfollowed by quantization. Various intra prediction modes may be madeavailable and parameters related to intra mode selection and otherparameters may be signaled in the bitstream. The various intraprediction modes, for example, may pertain to line position or positionsfor predicting samples, directions along which prediction samples areselected from predicting line or lines, and other special intraprediction modes.

For example, a set of intra prediction modes (interchangeably referredto as “intra modes”) may include a predefined number of directionalintra prediction modes. As described above in relation to the exampleimplementation of FIG. 1 , these intra prediction modes may correspondto a predefined number of directions along which out-of-block samplesare selected as prediction for samples being predicted in a particularblock. In another particular example implementation, eight (8) maindirectional modes corresponding to angles from 45 to 207 degrees to thehorizontal axis may be supported and predefined.

In some other implementations of intra prediction, to further exploitmore varieties of spatial redundancy in directional textures,directional intra modes may be further extended to an angle set withfiner granularity. For example, the 8-angle implementation above may beconfigured to provide eight nominal angles, referred to as V_PRED,H_PRED, D45_PRED, D135_PRED, D113_PRED, D157_PRED, D203_PRED, andD67_PRED, as illustrated in FIG. 9 , and for each nominal angle, apredefined number (e.g., 7) of finer angles may be added. With such anextension, a larger total number (e.g., 56 in this example) ofdirectional angles may be available for intra prediction, correspondingto the same number of predefined directional intra modes. A predictionangle may be represented by a nominal intra angle plus an angle delta.For the particular example above with 7 finer angular directions foreach nominal angle, the angle delta may be −3˜3 multiplies a step sizeof 3 degrees.

The above directional intra prediction may also be referred as singledirectional intra prediction, which is different from bi-directionalintra prediction (also referred as intra bi-prediction) described inlater part of the present disclosure.

In some implementations, alternative or in addition to the directionintra modes above, a predefined number of non-directional intraprediction modes may also be predefined and made available. For example,5 non-direction intra modes referred to as smooth intra prediction modesmay be specified. These non-directional intra mode prediction modes maybe specifically referred to as DC, PAETH, SMOOTH, SMOOTH_V, and SMOOTH_Hintra modes. Prediction of samples of a particular block under theseexample non-directional modes are illustrated in FIG. 10 . As anexample, FIG. 10 shows a 4×4 block 1002 being predicted by samples froma top neighboring line and/or left neighboring line. A particular sample1010 in block 1002 may correspond to directly top sample 1004 of thesample 1010 in the top neighboring line of block 1002, a top-left sample1006 of the sample 1010 as the intersection of the top and leftneighboring lines, and a directly left sample 1008 of the sample 1010 inthe left neighboring line of block 1002. For the example DC intraprediction mode, an average of the left and above neighboring samples1008 and 1004 may be used as the predictor of the sample 1010. For theexample PAETH intra prediction mode, the top, left, and top-leftreference samples 1004, 1008, and 1006 may be fetched, and thenwhichever value among these three reference samples that is the closestto (top+left−topleft) may be set as the predictor for the sample 1010.For the example SMOOTH_V intra prediction mode, the sample1010 may bepredicted by a quadratic interpolation in vertical direction of thetop-left neighboring sample 1006 and the left neighboring sample 1008.For the example SMOOTH_H intra prediction mode, the sample 1010 may bepredicted by a quadratic interpolation in horizontal direction of thetop-left neighboring sample 1006 and the top neighboring sample 1004.For the example SMOOTH intra prediction mode, the sample 1010 may bepredicted by an average of the quadratic interpolations in the verticaland the horizontal directions. The non-directional intra modeimplementations above are merely illustrated as a non-limiting example.Other neighboring lines, and other non-directional selection of samples,and manners of combining predicting samples for predicting a particularsample in a prediction block are also contemplated.

Selection of a particular intra prediction mode by the encoder from thedirectional or non-directional modes above at various coding levels(picture, slice, block, unit, etc.) may be signaled in the bitstream. Insome example implementations, the exemplary 8 nominal directional modestogether with 5 non-angular smooth modes (a total of 13 options) may besignaled first. Then if the signaled mode is one of the 8 nominalangular intra modes, an index is further signaled to indicate theselected angle delta to the corresponding signaled nominal angle. Insome other example implementations, all intra prediction modes may beindexed all together (e.g., 56 directional modes plus 5 non-directionalmodes to yield 61 intra prediction modes) for signaling.

In some example implementations, the example 56 or other number ofdirectional intra prediction modes may be implemented with a unifieddirectional predictor that projects each sample of a block to areference sub-sample location and interpolates the reference sample by a2-tap bilinear filter.

In some implementations, to capture decaying spatial correlation withreferences on the edges, additional filter modes referred to as FILTERINTRA modes may be designed. For these modes, predicted samples withinthe block in addition to out-of-block samples may be used as intraprediction reference samples for some patches within the block. Thesemodes, for example, may be predefined and made available to intraprediction for at least luma blocks (or only luma blocks). A predefinednumber (e.g., five) of filter intra modes may be pre-designed, eachrepresented by a set of n-tap filters (e.g., 7-tap filters) reflectingcorrelation between samples in, for example, a 4×2 patch and n neighborsadjacent to it. In other words, the weighting factors for an n-tapfilter may be position dependent. Taking an 8×8 block, 4×2 patch, and7-tap filtering as an example, as shown in FIG. 11 , the 8×8 block 1102may be split into eight 4×2 patches. These patches are indicated by B0,B1, B1, B3, B4, B5, B6, and B7 in FIG. 11 . For each patch, its 7neighbors, indicated by R0˜R7 in FIG. 11 , may be used to predict thesamples in a current patch. For patch B0, all the neighbors may havebeen already reconstructed. But for other patches, some of the neighborsare in the current block and thus may not have been reconstructed, thenthe predicted values of immediate neighbors are used as the reference.For example, all the neighbors of patch B7 as indicated in FIG. 11 arenot reconstructed, so the prediction samples of neighbors, for example aportion of B4, B5, and/or B6, are used instead.

In some implementation of intra prediction, one color component may bepredicted using one or more other color components. A color componentmay be any one of components in YCrCb, RGB, XYZ color space and thelike. For example, a prediction of chroma component (e.g., chroma block)from luma component (e.g., luma reference samples), referred to asChroma from Luma, or CfL), may be implemented. In some exampleimplementations, cross-color prediction many only be allowed from lumato chroma. For example, a chroma sample in a chroma block may be modeledas a linear function of coincident reconstructed luma samples. The CfLprediction may be implemented as follows:

CfL(α)=α×L ^(AC) +DC   (1)

where L^(AC) denotes an AC contribution of luma component, α denotes aparameter of the linear model, and DC denotes a DC contribution of thechroma component. The AC components, for example is obtained for eachsamples of the block whereas the DC component is obtained for the entireblock. To be specific, the reconstructed luma samples may be subsampledinto the chroma resolution, and then the average luma value (DC of luma)may be subtracted from each luma value to form the AC contribution inluma. The AC contribution of Luma is then used in the linear mode of Eq.(1) to predict the AC values of the chroma component. To approximate orpredict chroma AC component from the luma AC contribution, instead ofrequiring the decoder to calculate the scaling parameters, an exampleCfL implementation may determine the parameter α based on the originalchroma samples and signal them in the bitstream. This reduces decodercomplexity and yields more precise predictions. As for the DCcontribution of the chroma component, it may be computed using intra DCmode within the chroma component in some example implementations.

Turning back to intra prediction, in some example implementations,prediction of samples in a coding block or prediction block may be basedon one of a set of reference lines. In other words, rather than alwaysusing a nearest neighboring line (e.g., the immediate top neighboringline or the immediate left neighboring line of the prediction block asillustrated in FIG. 1 above), multiple reference lines may be providedas options for selection for intra prediction. Such intra predictionimplementations may be referred to as Multiple Reference Line Selection(MRLS). In these implementations, an encoder decides and signals whichreference line of a plurality of reference lines is used to generate theintra predictor. At the decoder side, after parsing the reference lineindex, the intra prediction of current intra-prediction block can begenerated by identifying the reconstructed reference samples by lookingup the specified reference line according to the intra prediction mode(such the directional, non-directional, and other intra-predictionmodes). In some implementations, a reference line index may be signaledin the coding block level and only one of the multiple reference linesmay be selected and used for intra prediction of one coding block. Insome examples, more than one reference lines may be selected togetherfor intra-prediction. For example, the more than one reference lines maybe combined, averaged, interpolated or in any other manner, with orwithout weight, to generate the prediction. In some exampleimplementations, MRLS may only be applied to luma component and may notbe applied to chroma component(s).

In FIG. 12 , an example of 4 reference-line MRLS is depicted. As shownin the example of FIG. 12 , the intra-coding block 1202 may be predictedbased on one of the 4 horizontal reference lines 1204, 1206, 1208, and1210 and 4 vertical reference lines 1212, 1214, 1216, and 1218. Amongthese reference lines, 1210 and 1218 are the immediate neighboringreference lines. The reference lines may be indexed according to theirdistance from the coding block. For example, reference lines 1210 and1218 may be referred to as zero reference line whereas the otherreference lines may be referred to as non-zero reference lines.Specifically, reference lines 1208 and 1216 may be reference as 1streference lines; reference lines 1206 and 1214 may be reference as 2ndreference lines; and reference lines 1204 and 1212 may be reference as3rd reference lines.

In some embodiments, for a specific coding block, coding unit,prediction block, or prediction unit that is intra coded, its intra modeneeds to be signaled by one or more syntax elements in the bitstream. Asdescribed above, the number of possible intra prediction modes may bevast, and there may be 62 intra prediction modes available: 56directional intra prediction modes, 5 non-directional modes, and onechroma from luma mode (e.g., only for chroma component). To signal theseintra prediction modes, a first syntax may be signaled to indicate whichnominal angle or non-directional mode is equal to the nominal mode of acurrent block. Then, if the mode of the current block is a directionalmode, a second syntax may be signaled to indicate which delta angle isequal to that of the current block. In some circumstances during videoencoding and/or decoding, there may be a strong correlation between theintra prediction mode of a current block and its neighboring blocks.

In various embodiments, this correlation may be exploited for designinga more efficient syntax for intra mode coding. In some implementations,the available intra prediction modes for the current block may be splitinto a plurality of intra prediction mode sets according to the intraprediction modes of its neighboring blocks. To get the intra predictionmode of the current block, firstly, a mode set index may be signaled toindicate the mode set index of the intra prediction mode for the currentblock; and secondly, a mode index may be signaled to indicate the indexof intra prediction mode within the mode set.

Here in various embodiments of the present disclosure, that “XYZ issignaled” may refer to that XYZ is encoded into a coded bitstream duringan encoding process; and/or, after the coded bitstream is transmittedfrom one device to another device, that “XYZ is signaled” may refer tothat XYZ is decoded/extracted from a coded bitstream during a decodingprocess.

For example, in some of the implementations described above, the numberof available intra prediction modes may include 62 different modes,including, for example, 56 directional intra prediction modes (e.g., 8nominal direction with 7 fine angles in each nominal direction), 5non-directional modes, and one chroma-from-luma mode (only for chromacomponents). Once an intra mode is selected during the coding processfor a particular coding block, coding unit, prediction block, orprediction, a signaling corresponding to the selected intra mode needsto be included in the bitstream. Signaling syntax(es) must be able todifferentiate all these 62 modes in some manner. For example, these 62modes may be signaled using a single syntax for 62 indices eachcorresponding to one mode. In some other example implementations, onesyntax may be signaled to indicate which nominal angle ornon-directional mode is used as a nominal mode in the current block, andthen, if the nominal mode of current block is a directional mode,another syntax may be additionally signaled to indicate which deltaangle is selected for the current block.

As various syntaxes pertaining to intra coding typically occupy a largeportion of the bitstream and that the intra mode selection must besignally frequently, e.g., at various coding levels, a reduction of thenumber of bits used for intra mode signaling becomes critical inimproving video coding efficiency. In practice, usage of the variousintra prediction modes may follow certain statistical patterns, and suchusage pattern may be utilized to design the indexing of the intra modesand signaling syntaxes such that the signaling efficiency can beenhanced. Further, some correlations may exist, on average, betweenintra mode selections from block to block. Such correlations may beobtained offline on a statistical basis and considered in the design ofthe syntax(es) for the signaling of selection of intra modes. The goalis to reduce, on average, the number of bits for the signaling syntaxelements in the coded bitstream. For example, some general statisticsmay indicate that there may be a strong correlation between an optimalintra prediction mode of a current block and its neighboring blocks.Such correlation may be exploited when designing the syntax(es) forintra mode coding.

In some embodiments, to improve video encoding/decoding performance, anoffset-based refinement for intra prediction (ORIP) may be used aftergenerating an intra prediction samples. When ORIP is applied, theprediction samples are refined by adding an offset value.

As shown in FIG. 13 , an intra prediction (1330) is performed based onreference samples. The reference samples may include samples from one ormore left reference lines (1312) and/or one or more top reference lines(1310). The offset-based refinement for intra prediction (ORIP) (1350)may generate an offset value using neighboring reference samples. Insome implementations, the neighboring reference samples for ORIP may bethe same set as the reference samples for intra prediction. In someother implementations, the neighboring reference samples for ORIP may bedifferent set as the reference samples for intra prediction.

In some implementations referring to FIG. 14A and FIG. 14B, ORIP may beperformed in 4×4 sub-block level. For each 4×4 sub-block (1471, 1472,1473, and/or 1474), the offsets are generated from its neighboringsamples. For example, for a first sub-block (1471), the offsets aregenerated from its top neighboring samples (P1, P2, P3, and P4 in 1420),left neighboring samples (P5, P6, P7, and P8 in 1410), and/or a top-leftneighboring sample (P0) (1401). In some implementations, top neighboringsamples may include both the top neighboring samples (P1, P2, P3, and P4in 1420) and the top-left neighboring sample (P0) (1401). In some otherimplementations, left neighboring samples may include both the leftneighboring samples (P5, P6, P7, and P8 in 1410) and the top-leftneighboring sample (P0) (1401).

The first sub-block (1471) includes 4×4 pixels, and each pixel of the4×4 pixels corresponds to predN being a N-th neighboring predictedsamples before refinement. For example, pred0, pred1, pred2, . . .pred16.

In various embodiments, the offset value of each pixel of a givensub-block may be calculated based on the neighboring samples accordingto a formula. The formula may be a pre-defined formula, or a formulaindicated by a parameter coded in a coded bitstream.

In some implementation referring to FIG. 14B, the offset value of k-thposition (offset(k)) of a given sub-block may be generated as follows:

$\begin{matrix}{{{offset}{}(k)} = {\left( {{\sum\limits_{n = 0}^{8}{W_{kn}*\left\{ {P_{n} - {pred_{k}}} \right\}}} + 32} \right) \gg 6}} & (2)\end{matrix}$ $\begin{matrix}{{pred\_ refined}_{k} = {{clip}3\left( {{pred_{k}} + {{offset}(k)}} \right)}} & (3)\end{matrix}$

W_(kn) is predefined weights for offset computation. Pn are values ofneighboring samples (e.g., P0, P1, P2, . . . , P8). predk are predictedvalues for pixels after the application of an intra prediction or otherprediction (e.g., inter prediction). pred refinedk are refined valuesfor pixels after the application of ORIP. clip3( ) is a clip3mathematical function. n is an integer from 0 to 8, inclusive. k is aninteger from 0 to 15, inclusive.

In some implementations, W_(kn) may be predefined and may be obtainedaccording to Table 1.

TABLE 1 Predefined weights for offset computation k W_(k0) W_(k1) W_(k2)W_(k3) W_(k4) W_(k5) W_(k6) W_(k7) W_(k8) 0 4 16 4 0 0 16 4 0 0 1 2 4 164 0 8 2 0 0 2 1 0 4 16 4 4 1 0 0 3 0 0 2 4 16 2 0 0 0 4 2 8 2 0 0 4 16 40 5 0 2 8 2 0 2 8 2 0 6 0 0 2 8 2 1 4 1 0 7 0 0 0 2 8 1 2 0 0 8 0 4 0 00 0 4 16 4 9 0 0 4 0 0 0 2 8 2 10 0 0 1 4 1 0 1 4 1 11 0 0 0 2 4 0 0 4 012 0 0 1 0 0 0 2 4 16 13 0 0 0 1 0 0 1 2 8 14 0 0 1 2 1 0 0 1 4 15 0 0 01 2 0 0 1 2

In some other implementations, the sub-block based ORIP may be appliedonly to a predefined set of intra-prediction modes and/or may bedifferently for luma and chroma depending on the intra-prediction mode.Table 2 shows one implementation of the sub-block based ORIP accordingto various intra-prediction modes and either luma or chroma channels.Taking luma channel as an example: when the prediction mode is either DCor SMOOTH, the ORIP is always ON and no additional signaling isrequired; when the prediction mode is HOR/VER and angle_delta equals to0, a block level signaling is required to enable/disable of the ORIP;and/or when the intra prediction mode is other mode, the ORIP is alwaysOFF and no additional signaling is required.

TABLE 2 Mode dependent ON/OFF of the proposed method ORIPIntra-prediction modes Luma Chroma DC ON OFF SMOOTH ON ON HOR/VER modewith angle_delta == 0 ON/OFF (signal) ON Other modes OFF OFF

Referring back to a second 4×4 sub-block (1473), due to its relativeposition with the first 4×4 sub-block (1471), the second sub-block's topneighboring samples may be some pixels of the first sub-block: thesecond sub-block's P1 may be the first block's pred12, the secondsub-block's P2 may be the first sub-block's pred13, the secondsub-block's P3 may be the first sub-block's pred14, and the secondsub-block's P4 may be the first sub-block's pred15. The secondsub-block's top-left neighboring sample (P0) may be the firstsub-block's left neighboring sample (P8).

In various embodiments, the available intra prediction modes or modeoptions for a current block being coded may be split into a plurality ofintra prediction mode sets. Each set may be assigned a mode set index.Each set may contain a number of intra mode prediction modes. The mannerin which the available intra prediction modes is split and ordered andthe intra prediction modes are ordered in each of the mode set may bedetermined at least in part according to the intra prediction modes usedby its neighboring blocks, based on correlation between intra predictionmodes between blocks. The intra prediction modes used by the neighboringblocks may be referred to as “reference intra prediction modes” or“reference mode”. Intra prediction mode for a particular unit may bedetermined and selected. The selection of the intra prediction mode maybe signaled. First, a mode set index may be signaled to indicate themode set index of the intra prediction mode set containing the selectedintra prediction mode. Secondly, a mode index (alternatively referred toas mode position index within a set) may be signaled to indicate anindex of selected intra prediction mode within the mode set.

The general implementations of intra prediction mode division andordering above and the specific examples below takes advantage ofstatistical effects and neighboring correlation to dynamically indexthese modes such that the design of syntaxes for signaling theirselection in the coded video bitstream can be optimized to improvecoding efficiency. For example, these implementations may help reducethe number of syntaxes for the signaling and help more efficient contextgeneration for entropy coding.

The various embodiments and/or implementations described in the presentdisclosure may be used separately or combined in any order. Further, aportion, all, or any partial or whole combinations of these embodimentsand/or implementations may be embodied as a portion of an encoder and/ora decoder, and may be implemented in hardware and/or software. Forexample, they may be hard coded in dedicated processing circuitry (e.g.,one or more integrated circuits). In one other example, they may beimplemented by one or more processors executing a program that is storedin a non-transitory computer-readable medium.

There may be some issues/problems associated with intra mode coding. Forexample, it may be very challenging for hardware implementation of intramode coding, particularly the construction process of intra mode listfor small block.

The present disclosure describes various embodiment for intra predictionmode coding in video coding and/or decoding, addressing at least one ofthe issues/problems discussed above, achieving an efficientsoftware/hardware implementation for improved intra mode coding. In someembodiment, the signaling and/or coding for intra prediction mode forsmall blocks may be implemented according to a pre-defined rule, so asto simplify syntax for efficient software/hardware implementation.

In various embodiment, referring to FIG. 15 , a method 1500 for intraprediction mode coding in video decoding, the method 1500 may include aportion or all of the following step: step 1510, receiving, by a devicecomprising a memory storing instructions and a processor incommunication with the memory, a coded video bitstream; step 1520,constructing, by the device, a list of intra modes for a block in thecoded video bitstream according to a pre-defined rule based on a size ofthe block; step 1530, dividing, by the device, the list of intra modesinto a plurality of intra mode sets for the block; step 1540,extracting, by the device from the coded video bitstream, a set indexindicating an intra mode set from the plurality of intra mode sets; step1550, extracting, by the device from the coded video bitstream, a modeindex indicating an intra prediction mode from the intra mode set; step1560, determining, by the device, the intra prediction mode for theblock based on the set index and the mode index; and/or step 1570,decoding, by the device, the coded video bitstream based on the intraprediction mode.

In various embodiments in the present disclosure, a size of a block (forexample but not limited to, a coding block, a prediction block, or atransform block) may refer to a width or a height of the block. Thewidth or the height of the block may be an integer in a unit of pixels.In various embodiments in the present disclosure, a size of a block mayrefer to an area size of the block. The area size of the block may be aninteger calculated by the width of the block multiplied by the height ofthe bock in a unit of pixels. In some various embodiments in the presentdisclosure, a size of a block may refer to a maximum value of a width ora height of the block, a minimum value of a width or a height of theblock, or an aspect ratio of the block. The aspect ratio of the blockmay be calculated as the width divided by the height of the block, ormay be calculated as the height divided by the width of the block.

In some implementations, a mode type of an intra mode may include atleast one of the following: a directional mode, an non-directional mode,a smooth mode (e.g., smooth, smooth_v, smooth_h), a DC mode, a PAETHmode, and/or a mode that is generating prediction samples according to agiven prediction direction. In some other implementations, in a loosecategorization, a directional mode may broadly include: any mode that isnot smooth (smooth, smooth_v, smooth_h), DC, or PAETH mode; and any modethat is generating prediction samples according to a given predictiondirection. In some other implementations, a non-directional mode mayinclude a smooth mode (e.g., smooth, smooth_v, smooth_h), a DC mode, aPAETH mode, and a luma-for-chroma mode. In some other implementations,in a loose categorization, a non-directional mode may broadly includeany mode that is not a directional mode.

In some implementations, available intra prediction modes for currentblock may be divided/split into a plurality of intra prediction modesets. To get the intra prediction mode of a current block, firstly, amode set index may be signaled to indicate the mode set index of theintra prediction mode for the current block; secondly, a mode index maybe signaled to indicate the index of intra prediction mode within themode set.

Here in various embodiments of the present disclosure, a “first” modeset does not only refer to “one” mode set, but refers to the “first”mode set with a smallest mode set index, a “second” mode set does notonly refer to “another” mode set, but refers to the “second” mode setwith a second smallest mode set index, and so on. For example, a numberof intra prediction mode sets may be indicated by M and a mode set indexmay range from, for example, 1 to M, or 0 to M-1. When the mode setindex ranges from 1 to M, a “first” mode set is the “first” mode setwith the mode set index being 1, a “second” mode set is the “second”mode set with the mode set index being 2, and so on. When the mode setindex ranges from 0 to M-1, a “first” mode set is the “first” mode setwith the mode set index being 0, a “second” mode set is the “second”mode set with the mode set index being 1, and so on.

Here in various embodiments of the present disclosure, that “XYZ issignaled” may refer to that XYZ is encoded into a coded bitstream duringan encoding process; and/or, after the coded bitstream is transmittedfrom one device to another device, that “XYZ is signaled” may refer tothat XYZ is decoded/extracted from a coded bitstream during a decodingprocess.

Here in various embodiments in the present disclosure, a “block” mayrefer to a prediction block, a coding block, a transform block, or acoding unit (CU).

Referring to step 1510, the device may be the electronic device (530) inFIG. 5 or the video decoder (810) in FIG. 8 . In some implementations,the device may be the decoder (633) in the encoder (620) in FIG. 6 . Inother implementations, the device may be a portion of the electronicdevice (530) in FIG. 5 , a portion of the video decoder (810) in FIG. 8, or a portion of the decoder (633) in the encoder (620) in FIG. 6 . Thecoded video bitstream may be the coded video sequence in FIG. 8 , or anintermediate coded data in FIG. 6 or 7 . The block may refer to a codingblock or a coded block.

Referring to step 1520, the device may construct a list of intra modesfor the block according to a pre-defined rule based on a size of theblock. Referring to step 1530, the device may divide the list of intramodes into a plurality of intra mode sets for the block. The pre-definedrule may be one of a set of pre-defined rules, and may be selected fromthe et of pre-defined rules based on the size of the block.

In some implementations, an intra mode list is constructed for a currentblock based on the pre-defined rules. After constructing the intra modelist, intra prediction modes are split into multiple intra predictionmode sets according to their corresponding index in the intra mode list.To signal the intra prediction mode of the current block, firstly, amode set index may be signaled to indicate the mode set index of theintra prediction mode for the current block; and secondly, a mode indexis signaled to indicate the index of intra prediction mode within themode set. The pre-defined rules may be different for different blocksizes, which may be indicated by a number of samples in the block.

In various embodiments, in response to the size of the block being equalto or greater than a first threshold: the list of intra modes comprisesa first sub-list being at a top of the list; and the first sub-listcomprises all non-directional intra prediction modes. For example, whenthe list of intra modes is divided into the plurality of intra modesets, the non-directional intra prediction modes may be divided into afirst intra mode set because the non-directional intra prediction modesare at the top of the list. In some implementations, non-directionalmodes may include a smooth mode (e.g., smooth, smooth_v, smooth_h), a DCmode, a PAETH mode, and a luma-for-chroma mode. In some otherimplementations, in a loose categorization, non-directional modes maybroadly include any mode that is not a directional mode.

In some implementations, for blocks with a block size being equal to orgreater than a first threshold (TH1), firstly, all the non-directionalmodes are added into the mode list. Secondly, an offset is added to thedirectional intra prediction modes of neighboring blocks to derive theintra prediction modes, and the derived intra prediction modes are addedto the intra mode list. Finally, after adding all the derived intraprediction modes, if the intra prediction mode set is still not full,the default modes are used to fill the remaining positions in the intramode list. For one example, the first threshold is 8×8. For anotherexample, the first threshold is 32 in a number of samples.

In some other implementations, in response to a neighboring block usinga directional intra prediction mode: the list of intra modes comprises asecond sub-list being next to the first sub-list in the list; and thesecond sub-list comprises multiple derived intra prediction modes basedon the directional intra prediction mode of the neighboring block. Forexample, when the list of intra modes is divided into the plurality ofintra mode sets, the multiple derived intra prediction modes based onthe directional intra prediction mode of the neighboring block may belikely divided into either a first intra mode set or a second intra modeset because the multiple derived intra prediction modes are right nextto the non-directional intra prediction modes from the top of the list.In some implementation, the neighboring block of a current block mayinclude a top (upper) block of the current block, a left block of thecurrent block, or both top (upper) and left blocks of the current block.

In some other implementations, the multiple derived intra predictionmodes comprises nine directional intra prediction modes by adding anoffset of [0, −1, +1, −2, +2, −3, +3, −4, +4] to the directional intraprediction mode of the neighboring block. For example, when thedirectional intra prediction mode of the neighboring block has a certaindirectional angle (x degrees) and the step size of directional angle is3 degrees, the multiple derived intra prediction modes may include 9directional intra prediction modes with directional angle of x, x±3,x±6, x±9, and x±12 degrees.

In another example, the available intra prediction modes for a currentblock is 61, including 5 non-directional modes and 56 directional modes.When neighboring blocks of the current block is coded with one or moredirectional intra prediction modes, firstly, 5 non-directional modes areadded into the mode list; secondly, 9 directional intra prediction modesare derived by adding an offset of [0, −1, +1, −2, +2, −3, +3, −4, +4]to each of the directional modes of the neighboring block. When thereare no directional modes for any neighboring blocks, the second step maybe skipped. For one example, if only one neighboring box is coded withone directional intra prediction mode, 9 directional intra predictionmodes are derived, so that 14 (=9+5) modes are added into the mode list,and then, the default modes are added into the mode list. For anotherexample, if two neighboring blocks are coded with two directional intraprediction modes, 18 (=9*2) directional intra prediction modes may bederived, so that 23 (=18+5) modes are added into the mode list, andthen, the default modes are added into the mode list. Under somecircumstances, if two neighboring blocks are coded with two directionalintra prediction modes, fewer than 18 directional intra prediction modesmay be derived after removing any duplicates between the two 9 deriveddirectional intra prediction modes. In an extreme circumstance, if twoneighboring blocks are coded with two identical directional intraprediction modes, only 9 directional intra prediction modes may bederived after removing the duplicates between the identical two 9derived directional intra prediction modes.

In some other implementations, the list of intra modes comprises a thirdsub-list being next to the second sub-list in the list; and the thirdsub-list comprises default intra prediction modes. In some otherimplementations, the default intra prediction modes comprises at leastone nominal directional intra prediction modes with zero delta angle.For example, the default intra prediction modes may include all othernominal directional intra prediction modes with zero delta angle whichis not already included in the above derived intra prediction modesbased on the directional intra prediction mode of the neighboring block.

In various embodiments, in response to the size of the block beingsmaller than a first threshold: the list of intra modes comprises afirst sub-list being at a top of the list; and the first sub-listcomprises all non-directional intra prediction modes. In someimplementations, in response to neighboring blocks using directionalintra prediction modes: the list of intra modes comprises a secondsub-list being next to the first sub-list in the list; and the secondsub-list comprises all directional intra prediction modes of theneighboring blocks. In some other implementations, the list of intramodes comprises a third sub-list being next to the second sub-list inthe list; and the third sub-list comprises default intra predictionmodes. In some other implementations, in response to a number of alldirectional intra prediction modes of the neighboring blocks beinglarger than 1: the directional intra prediction modes of the neighboringblocks in the list of intra modes have an order of angles from smallerto larger.

In some implementations, for blocks with a block size being smaller thana first threshold TH1, firstly, all the non-directional modes are addedinto the mode list; secondly, the directional intra prediction modes ofneighboring blocks are added to the intra mode list; and then, thedefault modes are used to fill the remaining positions of the intra modelist. In one example, the first threshold (TH1) is set to block size8×8. In another example, if there are two directional intra predictionmodes in neighboring blocks, the directional mode with smaller angles isfirstly added into the mode list. For example, vertical mode is 90degrees and horizontal mode is 180 degrees, and the angle of verticalmode is smaller than that of horizontal mode, so that vertical mode isfirstly added into the mode list.

In various embodiments, the in response to the size of the block beingsmaller than a first threshold: the list of intra modes comprisespre-defined default intra prediction modes. In some implementations, thepre-defined default intra prediction modes in the list of intra modeshave the following order: firstly, all non-directional intra predictionmodes, secondly, directional intra prediction modes with a delta angleof zero, thirdly, directional intra prediction modes with a delta angleof +2 or −2 times a step size, fourthly, directional intra predictionmodes with a delta angle of +1 or −1 times the step size, and/orfifthly, directional intra prediction modes with a delta angle of +3 or−3 times the step size. In some implementations, the step size may be adefault value of 3 degrees.

In some implementations, for blocks with a block size smaller than athreshold (TH1), the pre-defined default modes are used to fill all thepositions in the intra mode list. For one example, the pre-defineddefault modes may be listed below: firstly, all the non-directionalmodes are added into the mode list; secondly, directional modes withdelta angle equal to 0 are added into the mode list; thirdly,directional modes with delta angle equal to 2 and −2 are added into themode list; fourthly, directional modes with delta angle equal to 1 and−1 are added into the mode list; and/or fifthly, directional modes withdelta angle equal to 3 and −3 are added into the mode list.

In various embodiments, in response to the size of the block beingsmaller than a first threshold: the intra mode set indicated by the setindex belongs to N intra mode sets, wherein the N intra mode sets are ata top of the plurality of intra mode sets, and N is a positive integer.

In some implementations, for blocks smaller than a threshold (TH1), onlythe first N intra mode sets are allowed and signaled for the currentblock. N is a positive integer. In one example, N is set to 1,indicating only the first intra mode set is allowed and signaled for thecurrent block. In another example, N is set to 2, indicating only thefirst intra mode set and the second intra mode set are allowed andsignaled for the current block.

Embodiments in the disclosure may be used separately or combined in anyorder. Further, each of the methods (or embodiments), an encoder, and adecoder may be implemented by processing circuitry (e.g., one or moreprocessors or one or more integrated circuits). In one example, the oneor more processors execute a program that is stored in a non-transitorycomputer-readable medium. Embodiments in the disclosure may be appliedto a luma block or a chroma block; and in the chroma block, theembodiments may be applied to more than one color components separatelyor may be applied to more than one color components together.

The techniques described above, can be implemented as computer softwareusing computer-readable instructions and physically stored in one ormore computer-readable media. For example, FIG. 16 shows a computersystem (2600) suitable for implementing certain embodiments of thedisclosed subject matter.

The computer software can be coded using any suitable machine code orcomputer language, that may be subject to assembly, compilation,linking, or like mechanisms to create code comprising instructions thatcan be executed directly, or through interpretation, micro-codeexecution, and the like, by one or more computer central processingunits (CPUs), Graphics Processing Units (GPUs), and the like.

The instructions can be executed on various types of computers orcomponents thereof, including, for example, personal computers, tabletcomputers, servers, smartphones, gaming devices, internet of thingsdevices, and the like.

The components shown in FIG. 16 for computer system (2600) are exemplaryin nature and are not intended to suggest any limitation as to the scopeof use or functionality of the computer software implementingembodiments of the present disclosure. Neither should the configurationof components be interpreted as having any dependency or requirementrelating to any one or combination of components illustrated in theexemplary embodiment of a computer system (2600).

Computer system (2600) may include certain human interface inputdevices. Such a human interface input device may be responsive to inputby one or more human users through, for example, tactile input (such as:keystrokes, swipes, data glove movements), audio input (such as: voice,clapping), visual input (such as: gestures), olfactory input (notdepicted). The human interface devices can also be used to capturecertain media not necessarily directly related to conscious input by ahuman, such as audio (such as: speech, music, ambient sound), images(such as: scanned images, photographic images obtain from a still imagecamera), video (such as two-dimensional video, three-dimensional videoincluding stereoscopic video).

Input human interface devices may include one or more of (only one ofeach depicted): keyboard (2601), mouse (2602), trackpad (2603), touchscreen (2610), data-glove (not shown), joystick (2605), microphone(2606), scanner (2607), camera (2608).

Computer system (2600) may also include certain human interface outputdevices. Such human interface output devices may be stimulating thesenses of one or more human users through, for example, tactile output,sound, light, and smell/taste. Such human interface output devices mayinclude tactile output devices (for example tactile feedback by thetouch-screen (2610), data-glove (not shown), or joystick (2605), butthere can also be tactile feedback devices that do not serve as inputdevices), audio output devices (such as: speakers (2609), headphones(not depicted)), visual output devices (such as screens (2610) toinclude CRT screens, LCD screens, plasma screens, OLED screens, eachwith or without touch-screen input capability, each with or withouttactile feedback capability—some of which may be capable to output twodimensional visual output or more than three dimensional output throughmeans such as stereographic output; virtual-reality glasses (notdepicted), holographic displays and smoke tanks (not depicted)), andprinters (not depicted).

Computer system (2600) can also include human accessible storage devicesand their associated media such as optical media including CD/DVD ROM/RW(2620) with CD/DVD or the like media (2621), thumb-drive (2622),removable hard drive or solid state drive (2623), legacy magnetic mediasuch as tape and floppy disc (not depicted), specialized ROM/ASIC/PLDbased devices such as security dongles (not depicted), and the like.

Those skilled in the art should also understand that term “computerreadable media” as used in connection with the presently disclosedsubject matter does not encompass transmission media, carrier waves, orother transitory signals.

Computer system (2600) can also include an interface (2654) to one ormore communication networks (2655). Networks can for example bewireless, wireline, optical. Networks can further be local, wide-area,metropolitan, vehicular and industrial, real-time, delay-tolerant, andso on. Examples of networks include local area networks such asEthernet, wireless LANs, cellular networks to include GSM, 3G, 4G, 5G,LTE and the like, TV wireline or wireless wide area digital networks toinclude cable TV, satellite TV, and terrestrial broadcast TV, vehicularand industrial to include CAN bus, and so forth. Certain networkscommonly require external network interface adapters that attached tocertain general-purpose data ports or peripheral buses (2649) (such as,for example USB ports of the computer system (2600)); others arecommonly integrated into the core of the computer system (2600) byattachment to a system bus as described below (for example Ethernetinterface into a PC computer system or cellular network interface into asmartphone computer system). Using any of these networks, computersystem (2600) can communicate with other entities. Such communicationcan be uni-directional, receive only (for example, broadcast TV),uni-directional send-only (for example CANbus to certain CANbusdevices), or bi-directional, for example to other computer systems usinglocal or wide area digital networks. Certain protocols and protocolstacks can be used on each of those networks and network interfaces asdescribed above.

Aforementioned human interface devices, human-accessible storagedevices, and network interfaces can be attached to a core (2640) of thecomputer system (2600).

The core (2640) can include one or more Central Processing Units (CPU)(2641), Graphics Processing Units (GPU) (2642), specialized programmableprocessing units in the form of Field Programmable Gate Areas (FPGA)(2643), hardware accelerators for certain tasks (2644), graphicsadapters (2650), and so forth. These devices, along with Read-onlymemory (ROM) (2645), Random-access memory (2646), internal mass storagesuch as internal non-user accessible hard drives, SSDs, and the like(2647), may be connected through a system bus (2648). In some computersystems, the system bus (2648) can be accessible in the form of one ormore physical plugs to enable extensions by additional CPUs, GPU, andthe like. The peripheral devices can be attached either directly to thecore's system bus (2648), or through a peripheral bus (2649). In anexample, the screen (2610) can be connected to the graphics adapter(2650). Architectures for a peripheral bus include PCI, USB, and thelike.

CPUs (2641), GPUs (2642), FPGAs (2643), and accelerators (2644) canexecute certain instructions that, in combination, can make up theaforementioned computer code. That computer code can be stored in ROM(2645) or RAM (2646). Transitional data can also be stored in RAM(2646), whereas permanent data can be stored for example, in theinternal mass storage (2647). Fast storage and retrieve to any of thememory devices can be enabled through the use of cache memory, that canbe closely associated with one or more CPU (2641), GPU (2642), massstorage (2647), ROM (2645), RAM (2646), and the like.

The computer readable media can have computer code thereon forperforming various computer-implemented operations. The media andcomputer code can be those specially designed and constructed for thepurposes of the present disclosure, or they can be of the kind wellknown and available to those having skill in the computer software arts.

As a non-limiting example, the computer system having architecture(2600), and specifically the core (2640) can provide functionality as aresult of processor(s) (including CPUs, GPUs, FPGA, accelerators, andthe like) executing software embodied in one or more tangible,computer-readable media. Such computer-readable media can be mediaassociated with user-accessible mass storage as introduced above, aswell as certain storage of the core (2640) that are of non-transitorynature, such as core-internal mass storage (2647) or ROM (2645). Thesoftware implementing various embodiments of the present disclosure canbe stored in such devices and executed by core (2640). Acomputer-readable medium can include one or more memory devices orchips, according to particular needs. The software can cause the core(2640) and specifically the processors therein (including CPU, GPU,FPGA, and the like) to execute particular processes or particular partsof particular processes described herein, including defining datastructures stored in RAM (2646) and modifying such data structuresaccording to the processes defined by the software. In addition or as analternative, the computer system can provide functionality as a resultof logic hardwired or otherwise embodied in a circuit (for example:accelerator (2644)), which can operate in place of or together withsoftware to execute particular processes or particular parts ofparticular processes described herein. Reference to software canencompass logic, and vice versa, where appropriate. Reference to acomputer-readable media can encompass a circuit (such as an integratedcircuit (IC)) storing software for execution, a circuit embodying logicfor execution, or both, where appropriate. The present disclosureencompasses any suitable combination of hardware and software.

While the particular invention has been described with reference toillustrative embodiments, this description is not meant to be limiting.Various modifications of the illustrative embodiments and additionalembodiments of the invention will be apparent to one of ordinary skillin the art from this description. Those skilled in the art will readilyrecognize that these and various other modifications can be made to theexemplary embodiments, illustrated and described herein, withoutdeparting from the spirit and scope of the present invention. It istherefore contemplated that the appended claims will cover any suchmodifications and alternate embodiments. Certain proportions within theillustrations may be exaggerated, while other proportions may beminimized. Accordingly, the disclosure and the figures are to beregarded as illustrative rather than restrictive.

What is claimed is:
 1. A method for intra prediction mode coding invideo decoding, the method comprising: receiving, by a device comprisinga memory storing instructions and a processor in communication with thememory, a coded video bitstream; constructing, by the device, a list ofintra modes for a block in the coded video bitstream according to apre-defined rule based on a size of the block; dividing, by the device,the list of intra modes into a plurality of intra mode sets for theblock; extracting, by the device from the coded video bitstream, a setindex indicating an intra mode set from the plurality of intra modesets; extracting, by the device, from the coded video bitstream, a modeindex indicating an intra prediction mode from the intra mode set;determining, by the device, the intra prediction mode for the blockbased on the set index and the mode index; and decoding, by the device,the coded video bitstream based on the intra prediction mode.
 2. Themethod according to claim 1, wherein: in response to the size of theblock being equal to or greater than a first threshold: the list ofintra modes comprises a first sub-list being at a top of the list; andthe first sub-list comprises all non-directional intra prediction modes.3. The method according to claim 2, wherein: the first threshold is 8×8;or the first threshold is 32 in a number of samples.
 4. The methodaccording to claim 2, wherein: in response to a neighboring block usinga directional intra prediction mode: the list of intra modes comprises asecond sub-list being next to the first sub-list in the list; and thesecond sub-list comprises multiple derived intra prediction modes basedon the directional intra prediction mode of the neighboring block. 5.The method according to claim 4, wherein: the multiple derived intraprediction modes comprises nine directional intra prediction modes byadding an offset of [0, −1, +1, −2, +2, −3, +3, −4, +4] to thedirectional intra prediction mode of the neighboring block.
 6. Themethod according to claim 4, wherein: the list of intra modes comprisesa third sub-list being next to the second sub-list in the list; and thethird sub-list comprises default intra prediction modes.
 7. The methodaccording to claim 6, wherein: the default intra prediction modescomprises at least one nominal directional intra prediction modes withzero delta angle.
 8. The method according to claim 1, wherein: inresponse to the size of the block being smaller than a first threshold:the list of intra modes comprises a first sub-list being at a top of thelist; and the first sub-list comprises all non-directional intraprediction modes.
 9. The method according to claim 8, wherein: inresponse to neighboring blocks using directional intra prediction modes:the list of intra modes comprises a second sub-list being next to thefirst sub-list in the list; and the second sub-list comprises alldirectional intra prediction modes of the neighboring blocks.
 10. Themethod according to claim 9, wherein: the list of intra modes comprisesa third sub-list being next to the second sub-list in the list; and thethird sub-list comprises default intra prediction modes.
 11. The methodaccording to claim 9, wherein: in response to a number of alldirectional intra prediction modes of the neighboring blocks beinglarger than 1: the directional intra prediction modes of the neighboringblocks in the list of intra modes have an order of angles from smallerto larger.
 12. The method according to claim 1, wherein: in response tothe size of the block being smaller than a first threshold: the list ofintra modes comprises pre-defined default intra prediction modes. 13.The method according to claim 12, wherein: the pre-defined default intraprediction modes in the list of intra modes have the following order:firstly, all non-directional intra prediction modes, secondly,directional intra prediction modes with a delta angle of zero, thirdly,directional intra prediction modes with a delta angle of +2 or −2 timesa step size, fourthly, directional intra prediction modes with a deltaangle of +1 or −1 times the step size, and fifthly, directional intraprediction modes with a delta angle of +3 or −3 times the step size. 14.The method according to claim 13, wherein: the step size is 3 degrees.15. The method according to claim 1, wherein: in response to the size ofthe block being smaller than a first threshold: the intra mode setindicated by the set index belongs to N intra mode sets, wherein the Nintra mode sets are at a top of the plurality of intra mode sets, and Nis a positive integer.
 16. The method according to claim 15, wherein: Nis one of 1 or
 2. 17. An apparatus for intra prediction mode coding invideo decoding, the apparatus comprising: a memory storing instructions;and a processor in communication with the memory, wherein, when theprocessor executes the instructions, the processor is configured tocause the apparatus to: receive a coded video bitstream; construct alist of intra modes for a block in the coded video bitstream accordingto a pre-defined rule based on a size of the block; divide the list ofintra modes into a plurality of intra mode sets for the block; extract,from the coded video bitstream, a set index indicating an intra mode setfrom the plurality of intra mode sets; extract, from the coded videobitstream, a mode index indicating an intra prediction mode from theintra mode set; determine the intra prediction mode for the block basedon the set index and the mode index; and decode the coded videobitstream based on the intra prediction mode.
 18. The apparatusaccording to claim 17, wherein: in response to the size of the blockbeing equal to or greater than a first threshold: the list of intramodes comprises a first sub-list being at a top of the list; and thefirst sub-list comprises all non-directional intra prediction modes. 19.A non-transitory computer readable storage medium storing instructions,wherein, when the instructions are executed by a processor, theinstructions are configured to cause the processor to: receive a codedvideo bitstream; construct a list of intra modes for a block in thecoded video bitstream according to a pre-defined rule based on a size ofthe block; divide the list of intra modes into a plurality of intra modesets for the block; extract, from the coded video bitstream, a set indexindicating an intra mode set from the plurality of intra mode sets;extract, from the coded video bitstream, a mode index indicating anintra prediction mode from the intra mode set; determine the intraprediction mode for the block based on the set index and the mode index;and decode the coded video bitstream based on the intra prediction mode.20. The non-transitory computer readable storage medium according toclaim 19, wherein: in response to the size of the block being equal toor greater than a first threshold: the list of intra modes comprises afirst sub-list being at a top of the list; and the first sub-listcomprises all non-directional intra prediction modes.